[Bug]: Add more error handling in planner page

### What happened?

When database or backend is down, we only send a toast saying something like "Internal Server Error".

### What is the expected behaviour?

We also to add a clear message that we couldnt find any programs when user tries to search for a program, or a course.

So :
- Add toast in Programs selector
- Add toast in CoursesList

### Steps to reproduce

You can stop the backend (locally ofc)

### Screenshots and additional context

_No response_

Search the frontend for the Programs selector and CoursesList components, since the issue names those as the places needing changes. Run the app locally with the backend stopped and try searching for a program or course. Done means those flows show a clear toast that no programs or courses could be found instead of only a generic internal server error.

Rédigé par le modèle d'indexation à partir du texte de l'issue.